Storage tube



Oct. 31, 1950 J PIERCE 2,527,652

' STORAGE TUBE Filed Jan. 29, 1948 FIG. I

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0F slam/.8 a! 1' /7 774m 0/? MESH urm 0F 7 CONDUCT, co/vnucruvc MATERIAL /METAL Lame 5/ 14 I //B FIG. 4 MATERIAL mus/Na man/err or BOMBARDMENT j 5 MATERIAL POSSEJSING PROPERTY OF BOMB! RDMENT INDUCED CONDUCT! VI 7) INVENTOR J. R. PIERCE B) I ATTORNEY Patented Oct. 31, 1950 STORAGE TUBE John a. Pierce, Millburn, N. 1., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application January 29, 1948, Serial No. 5,043

i 8 Claims. (Cl. 250-164) This invention relates to storage devices and more particularly to devices of this character utilizing electron beams for storing and reproducing electrical signals. 7

It is an object of this invention to utilize in storage tubes of the electron beam type materials exhibiting the property of electron bombardment induced conductivity. 7

It is another object of this invention to reduce, in storage tubes of this type, the distortions produced by stray secondary electrons.

In th copending applications of D. E. Wooldridge, Serial No. 747,888, filed May 14, 1947, and K. G. McKay, Serial No. 789,667, filed December 4, 1947, there are disclosed various materials which exhibit the property known as bombardment induced conductivity. Each of these materials (such' as, for example, diamond, zinc sulphide, magnesium oxide, silicon carbide and stibnite) is normally an insulator, but when it is struck by electrons (or other particles, such as alpha or beta particles, for example) it becomes conducting if at the time an electric field exists between opposite surfaces of the insulator. The bombarding particles penetrate the insulator, causing a disruptive separation of the positive and negative charges specific to the atoms which are afiected by the bombarding particles. These charges are drawn toward the electrodes producing the electric field and this motion of charges constitutes a conduction current which is in many cases greatly in excess of the currentof the bombarding particles.-

Diamond is a favored solid insulator for this Work (although other materials such as, for example, others mentioned in the Wooldridge and McKay applications can be used) because it can easily be obtained without sufiicient impurities or imperfections to afiect this high insulation re sistance or its conducting properties under bombardment. The carbon atoms therein consist each of a nucleus exhibiting fixed units of positive charge, to which two electrons are tightly bound. This core is surrounded by four valence electrons. The carbon atoms are held together by electron pair bonds between adjacent atoms. The insulation resistance is high because the electron bonds are very tight. As a result of this tightness, very few electrons are displaced from their bonds by thermal agitation. This is not'the case in, for example, metals, where a large number of electrons are continuously being displaced by thermal agitation and are relatively free to wander through the metal, this, under normal conditions, constituting the usual current in a metallic ,conducfting medium.

When electron bombardment removes a valence electron from its bonds in an insulating target, producing a deficienc of one electron in the atomic structure immediately afiected, this localized electron deficiency is called a hole. Under an applied electric field the arrangement of the electrons is changed, and the location of any given hole will change. As a consequence, the hole can be conveniently regarded as a positive particle which is free to move under the influence of the field. Similarly, the electron freed from the bond in question constitutes a negative particle which is free to move under the influence of the electric field. If there is no applied field, any free electron or positive hole moves in accordance with thermal agitation and consequently has a completely random motion. Under an applied electric field, there is .a directional motion superimposed on the random one. The order of mobility of the electrons in diamond is of the order of 1,000 centimeters per second for a field of one volt per centimeter. For a field of 10 volts per centimeter the velocity therefore is 10" centimeters per second. For a diamond crystal one millimeter thick, the transit time therefore is 10* seconds. Themobility of the electrons is afiected by the numberof traps, that is, thepres ence of foreign atoms or imperfections in the crystal. If an electron gets into a trap, it takes a greater or less amount of time to get out, depending upon the thermal energy required. Further information on traps and other characteristics of diamond crystals is given in the Wooldridge and McKay applications referred to above.

In accordance with the present invention, there is provided a storage tube of the electron beam type includingan electron target embodying material which exhibits the property of electron bombardment induced conductivity. Diamond is the preferred material for reasons given above. More specifically, the storage tube includes a target comprising a layer. of diamond or other material exhibiting the property of electron bombardment induced conductivity coated on one side with As the electron beam scans the target, the various elemental areas thereof are rendered successively conductive and the signals are stored as a charge distribution along the path the beam has swept'out. In reproducing the signals, the beam is made to retrace its previous path.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof in which:

Fig. 1 is a schematic representation of a cathode ray storage tube of this invention together with certain of its associated circuits and auxiliary equipment;

Fig. 2 is a schematic view showing, in greatly enlarged form, a portion of the target structure and associated conductingelectrode of the tube shown in Fig. 1;

Fig. 3 is a schematic circuit diagram to aid in the understanding of the invention; and

Fig. 4 shows a modification of the target structure and associated conducting electrode of the device shown in Fig. 1.

Referring more particularly to the drawing, Fig. 1 shows, by way .of example to illustrate the invention, a cathode ray storage tube. I!) employing a target ll containing material exhibiting .the property of electron bombardment induced conductivity. The tube l9 comprises an evacuated container I2 enclosing the target M, an electron gun 3 for generating, focussin and accelerating a beam of high velocity electrons toward this target, a mesh grid member 14 adjacent the target I I, and two sets of electrostatic deflecting :plates l and 13 for causing the beam of electrons to scan a number of elemental areas in turn of a desired field on the target I l. The target I I, which will be described below in greater detail with reference to Fig. 2, comprises a layer of material I! which exhibits the property of electron bombardment induced conductivity and which is coated on the side remote from the electron gun with a thin conducting metal layer [8.

A resistor I9 and a source of direct polarizing potential are connected in series between the metal layer [8 and the mesh grid member M. A signal utilization device 2| is connected, through a switch 22, across the resistor 19.

The electron gun l3 preferably comprises a cathode 23, a control electrode or member 24, a first anode member 25, and a second and final anode comprising a cylindrical member 26 and a coating 27 of conducting material on the inside walls of the envelope [2 extending from the region of the cylinder 26 to the region of the target H. The control electrode 24 is connected to the cathode 23 through a source 29 and a resistor 30 to place a negative bias on the control electrode.

Any suitable source 33 can be utilized to heat the cathode 23. The first anode 2-5 and the final anode 26, 21 are placed at appropriate positive potentials with respect to the cathode 23 by means of the source 3| and the source 32. For example, the final anode 2B, 21 can be from 1,000 to 10,000 or more volts positive with respect to the cathode and the first anode 25 can have an appropriate lower positive voltage for proper focussing. The negative terminal of the source 3| is connected to the cathode 23 and the positive terminal thereof is connected to the first anode 25, while the negative terminal of the source 32 -is connected to the positive terminal of the source 3| and the positive terminal of source 32 ,is connected to the second anode 26,. 21 and to ground. Batteries have, for convenience, been not been drawn to scale.

shown in the drawing but it is to be understood that any other means of producing direct voltages can be used instead.

Reference will now be made to Fig. 2 which shows in enlarged scale a portion of the target II and its adjacent grid member I4. Fig. 2 has As mentioned above, the target I 1 comprises a thin layer ll of insulating material which exhibits the property of electron bombardment induced conductivity and which has a thin metal coating IS on the surface thereof remote from the gun l3. By way of example, the layer l'l is a very thin out of diamond or a simulated sheet of diamond formed by a crystalline layer (preferably one particle thick) of diamond chips or diamond dust. Alternatively, the layer I! can 'be of any other suitable material exhibiting this desired property. The layer I! can be of the order of a millimeter thick, for example. The layer I8 is of any suitable materialsuch as gold, silver, platinum or aluminum,.for example.

The high'velocity beam produced by the gun l3 is deflected in a desired manner (such as line by line) over the surface of the target H by means of appropriate potentials applied to the pairs of deflecting plates l5 and It by electro static sweep circuits (not shown). If, for example, the signal to be stored and later reproduced is a signal formed by the scanning of a complete frame of a television object, suitable sweep circuits of the cyclically recurring type can be used. As an example of a suitable sweep circuit of this type, reference is made to Patent 2,178,464, issued October 31, 1939, to M. W. Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable for television. Two sweep circuits are, of course, used, one for frame scanning and one for line scanning, and connections from these sweep circuits are made to the pairs of plates l5 and I6 by means of coupling condensers 40, 4|, 42 and 43, respectively, of about 1 microfarad capacity each. Coupling resistances 44 and 45, of the order of many megohms each, are respectively connected across the pairs of plates l5 and 16. The mid-points of the resistances 44 and 45 are connected to the positive terminal of the source 32 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potential of the anodes 26, 21. This relationship is maintained to avoid changes in the sensitivity of the deflecting system and the consequent distortion of the image which would otherwise result. For a more complete description of the advantages of balanced sweep circuits for use with cathode ray television tubes, reference is made to the abovementioned Baldwin patent and also to Patent 2,209,199, issued July 23, 1940, to Frank Gray. Since the signal being stored has associated therewith the usual frame and line frequency synchronizing signals together with blanking signals, the usual television synchronizing circuits can beused to initiate starting of each line of the scannin in turn and to blank the beam during its fly-back operation between lines. If it is desired to record a whole television signal, two tubes l9 like those shown in Fig. 1 can be used, each connected so as to record and reproduce during alternate frame periods in a manner similar to that disclosed in a copending application of R. E. Graham, Serial No. 5,010, filed January The source of input signals 48 is connected across the resistor l9. A suitable switch 49 is shownfconnected in one of the leads between the source'48 and the resistorl9, the operation of the switch 49 being synchronized; by 'any'suitable means, with that of the switch 22 in such a' manner that when the switch-49 isjin a closed position the switch 22 is man open position, and vice versa. If two tubes H] are used, the sweep circuits are operated continuously so. that each tube recordsduring one frame perio'd andreproduces during the next. 1

a The operation of the device shown in Figfil will now be described. Assume that the signal to be stored on the layer 11 is a television video signal formed by scanning one frame. During the recording stage the switch member 49 is closed as shown on the drawing, while the switch 22 is open. The electron beam produced by the gun I3 is caused to scan, through the mesh grid 14. the uncoated side of the layer l7 and at the same time the signal to be stored is applied across the resistor I!) which is connected, in series with the source 20, between the surface 18 and the grid. As the electron beam scans the surface of the layer H, the various elemental areas thereof are rendered successively conductive, .1

and the signals are stored as a charge distribu-. tion along the path the beam sweeps out. The portion of the surface on which the beam falls thus attains substantially the same potential as the backing plate or layer l8. As the layer I1 is conducting Where the beam strikes it and the surface has the same potential as the backing plate, the voltage gradient at the surface where the beam falls due to the signal is Hence, a charge a' per unit area is induced on the surfacewhich is where e is the dielectric constant of vacuum which is 8.85 10-- farads per meter. -;[nreading the signal, the switch 49 is opened and the switch 22 is closed, and the output is applied to, the signal utilization device 2! which may be, for example, a cathode ray oscilloscope, or another tube of the type shown in Fig. 1 if it is desired to impart afurther delay .to the signal,

' or'anylother device utilizing the reproduced signals which are delayed replicas of the original signals. In the reproducing step the electron beam is made to retrace the path which it took in writing the signalon'the layer. IT. This synchronization is accomplished by any suitable mechanical or electronic means. Suppose the resistance l9 has such a low value that the voltage drop caused by the stored charge flowing through it is very small. Then, as the beam sweeps along the path, the surface of the material I! becomes conducting and the charge 0' flows through it.

Fig. 3 shows an equivalent circuit by means of which the amount of charge flowing through the resistance H] can be deduced. In Fig. 3, C1 is the capacitance per unit area between the grid member [4 and the surface of the material 11. Condenser C2 is the capacitance per unit area across the layer l1. When the electron beam strikes the material I! it is equivalent to closing .to produce bombardment induced conductivity voltage V2 across Cz-isequal and opposite to the voltage V1 across C1 and which is indicated in the claims.

age. appears across the capacitance C1; the charge left on capacitance C1 is then Her e d is the distance from the grid H! to the face of the target [1 adjacent it, t is the thickness of the material I1, and (61/6) is the ratio of the'dielectric constant of the material I! to the dielectric constant of vacuum.

Suppose the electron beam has a width w meters and travels at a rate of 1; meters per sec.- ond. Then the area swept out per second is through the resistor I9 I=Aa1=wva1 This will cause a voltage output Thus it follows from the above that the current can be expressed in terms of the voltage Vs applied in storing the signal as follows:

The current flow ewvR , l-( fl l/dl It is not necessary that the electron beam used strikethe free surface of the layer H. In Fig. 4, the electron beam passes, through the layer 50 which is'either a very thin layer of conducting material or which is a deposited grid or mesh member, and the grid member M of Fig. 1 is replaced in function by a plane electrode 5| spaced from the free surface of the layer [1. A direct current source 52 can produce a biasing voltage in either direction, as it has been found that a biasusually'gives an improved operation in connection with the operation of bombardment induced conductivity.

' Various other modifications can be made in 1 the embodiment described above Without departing-from the'spirit of the invention the scope of The specific potentials applied to the various elements are herein given merely by way of example and it is to be understood that their values may be made materially difierent without changing the general method of operation of the devices described herein. For example, the type of crystal material used and its thickness materially aifects the operating potentials required.

What is claimed is:

1. A recorder and reproducer of signalscomprising a target for electrons including a layer of electrically insulating material which possesses the property of becoming an electrical conductor when bombarded with electrons, an electrode member adjacent said target, electric circuit means for applying between said layer and said electrode a signal voltage to be stored, means for forming a beam of electrons and for causing it 7 tdistrike said'layer as said signals are being ap plied thereto, thereby forming a series of charges on said layer, and means for removing said charges at a later time to control the production of signals.

2. A device for the storage of electrical signals comprising a target forelectrons including a layer of material which is normally electrically insulating but which has the property of becommg electrically conducting when bombarded with electrons, a metal coating on one surface of said layer, a conducting electrode member on the side of said layer remote from said coating but spaced from said layer, and electric circuit means for applying a signal voltage to be stored between said coating and said electrode member.

3. A device for the storage of electrical signals comprising a target for electrons including a layer of material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with-electrons, a metal coating on one surface of said layer, a conducting electrode member on the side of said layer remote from said coating but Spaced from said layer, and electric circuit means for applying asignal voltage to be stored between said coating and said electrode member,

" said circuit meansincluding a biasing source of potential 1. A device for the'storage of electrical signals comprising a target for electrons including a layer of material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electronsa metal coating on onesurface of said layer, a conducting electrode member on the side of said layer remote from said coating but spaced from said layer, electric circuit means for applying a signal voltage to be stored between said coating and said electrode member, said connecting electrode comprising a mesh grid member, and means for bombarding said uncoated surface of said layer with a beam of electrons through said mesh grid member.

5. A device for the storage of electrical signals comprising a target for electrons including a layer of material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, a metal coating on one surface of said layer, a conducting electrode member on the side of said layer remote from said coating but spaced from said layer, electric circuit means for applyinga signal voltage to be stored between said coating and said electrode 'member, said layer being capable of transmitting electrons, and means for forming a beam of electrons and for directing it through said layer.

6. A device for the storage of electrical signals comprising a target for electrons including material which is normally electricallyinsulat ing but which has the property of becoming electrically conducting when bombarded with electrons, a metallic member on one face of said target, a second metallic member adjacent the opposite face, of said target but spaced therefrom, electriccircuit means for applying a signal voltage to be stored between said two metallic members, and means for scanning said target with a beam of electrons.

7. A device for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, a metallic member on one face of said target, a second metallic member adjacent the opposite face of said target but spaced therefrom, electric circuit means for applying a signal voltage to be stored between said two metallic members, and means for scanning said target with a beam of electrons, said last-mentioned means being so aligned with respect to said target that it passes through said spaced conducting member to strike said target.

8. A device for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, a metallic member on one face of said target, a second metallic member adjacent the opposite face of said target but spaced therefrom, electric circuit means for applying a signal voltage to be stored between said two metallic members, and means for scanning said target with a beam of electrons, said last-mentioned means be ing so aligned with respect to said target that the beam passes through said metallic member on said target to strike said material.

JOHN R. PIERCE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

